Application Security Testing: An Integral Part of DevOps
Owner-draw (or self-draw) controls are a little easier. Here, Windows sets up the device context for you. It also fills in a structure that gives you the rectangle that the control occupies, the state the control is in, and flags to say how much drawing you need to do. You still need to do all the drawing yourself, but with all this information handed to you, it is not such a chore. In particular, for controls like list boxes and ListView controls, Windows will go line by line through the control and ask you to draw each individual item; you don't need to know how to draw the entire control. To make things easy for owner-draw control writers, Windows also provides special drawing functions, like DrawFrameControl. Owner-draw is available for static, button, combo box and list box controls, but not for edit controls. It is also available for the ListView and Tab common controls. However, for ListView controls, owner draw only works in report mode and you have to draw the entire item (including all the sub items) in one go.
For Windows standard controls (buttons, edit boxes etc), one can also use the WM_CTLCOLOR family of notification messages to make some simple changes, usually just to the colours used. Windows sends WM_CTLCOLOR messages during the actually painting cycle of the control to give you the chance to make changes to the device context and nominate a brush to use for the background. You are somewhat limited in what you can do here, but the big advantage is that the control still does most of the work itself; you do not need to tell it how to draw itself. However, these messages are only available for the standard Windows controls; the Window Common controls do not support WM_CTLCOLOR.
Custom DrawWindows common controls add yet another method of customising called custom draw. This is different to owner draw, although some programmers confuse the two. It is available for ListView, TreeView, ToolTip, Header, TrackBar, Toolbar and Rebar controls.
NM_CUSTOMDRAW NotificationsWhen the control first starts to paint itself, in response to a WM_PAINT, you receive a NM_CUSTOMDRAW notification message, with the draw stage set to CDDS_PREPAINT. If you don't handle this yourself that will be the end of it, as the default message handler will just tell the control to carry on with default drawing and not to interrupt you again.
If you do handle the message, you have a chance to do a bit of painting yourself if you want. You can then set a flag in the return result that says whether you want the control to do its default painting; you usually will. You can also set flags to say whether you want to receive further notifications. You can ask the control to send you a WM_CUSTOMDRAW notification for the CDDS_POSTPAINT draw stage when the control has finished drawing (you can then do some extra drawing yourself). You can also say that you want to get notifications for the CDDS_ITEMPREPAINT draw stage for each item drawn.
Similarly, when you get each WM_CUSTOMDRAW notification for the CDDS_ITEMPREPAINT draw stage, you can set up the colours to use, make changes to the device context including font changes, and maybe do some drawing yourself. You can then say whether you want the default painting for the item and whether you want to receive a CDDS_ITEMPOSTPAINT when the item drawing is finished.
If you are in report mode, you can also ask for individual notifications for each subitem. These have the draw stage set to (CDDS_ITEMPREPAINT | CDDS_SUBITEM), and again you can fiddle with the device context, do your own drawing and/or let the control do some drawing and optionally receive a (CDDS_ITEMPOSTPAINT | CDDS_SUBITEM) drawing stage message at the end of each subitem.
NOTE: subitem notifications are only available for custom controls V4.71 (that is, for IE4.0 or Windows 98) or later, so please ensure that you check the version of COMCTL32 you have on your machine if you intend to use custom draw controls.
Here is a diagram showing the general flow of NM_CUSTOMDRAW notification messages for a list control with two items, each with two sub-items (columns):
Why Custom DrawOK. That's a lot of notification messages. But what's in it for me? Well, lets compare the advatnages of custom draw over owner draw for ListView controls:
|Owner Draw||Custom Draw|
|Only works with report-view style||Works for any style|
|Must do all the drawing yourself||Can choose you own drawing, default drawing of combinations of both. You can also change colours and fonts for default drawing.|
|Must draw the entire line (including subitems) in one go||Can handle sub items individually|
The price one pays for this flexibility is some complexity in writing the handler for the NM_CUSTOMDRAW. All the various drawing stages come through the one handler. It has to understand how to behave for each of the draw stages, extract the required information from the NMLVCUSTOMDRAW struct, and set the correct flags so that it receives subsequent notification messages correctly.
But things are more complicated than this. Some of the information in the NMLVCUSTOMDRAW struct is only relevant during certain draw-stage notifications. For example, Windows only sets the iSubItem value for the sub items notifications; it has rubbish values for the other notifications message. Similarly, Windows only fills in the RECT in the struct with valid values for some draw stages (depending on the version of common controls you have). Experimental evidence shows that even this is not completely correct; in fact only some of the values are valid during some drawing stages and others are not.
To ease the burden on the working class programmer, I have written a class called CListCtrlWithCustomDraw that does a lot of the housekeeping work for you, and uses simple virtual functions that you can override to provide the functionality you require. You do not need to worry about setting the correct flags, or unpacking information from structures and so on. In my next article, I'll present not only this example code, but I'll also walk you through the specifics on how exactly it all works.